ASoC: tfa9879: Fix return value check in tfa9879_i2c_probe()

In case of error, the function devm_kzalloc() returns NULL
not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org